3.03.0DATA TRANSMISSIONDATA TRANSMISSION

3.1 Analog & Digital Transmission3.1 Analog & Digital Transmission

3.2 Bandwidth3.2 Bandwidth3.3 Noise & Attenuation3.3 Noise & Attenuation3.4 Transmission Media3.4 Transmission Media3.5 Data Communication Interface3.5 Data Communication Interface3.6 Operation & use of multiplexors 3.6 Operation & use of multiplexors

Analog & Analog & DigitalDigital Transmission Transmission

AnalogAnalog - - InformationInformation changes continuously and can take changes continuously and can take

on many different values. on many different values. - An analog clock’s hands move constantly, - An analog clock’s hands move constantly,

displaying time on a continuous scale. displaying time on a continuous scale. DigitalDigital - InformationInformation is characterized by discrete states. is characterized by discrete states. - A light bulb, for example, is on or off. A light bulb, for example, is on or off. - A digital clock represents the time in one-A digital clock represents the time in one-

minute intervals and doesn’t change its minute intervals and doesn’t change its numbers again until the next minute. numbers again until the next minute.

Binary, Analog & Digital dataBinary, Analog & Digital data

BinaryBinary- Pertaining to a number system that has just two Pertaining to a number system that has just two

unique digits. (0,1)unique digits. (0,1)- Example : Example : binarybinary or or base-twobase-two numeral system numeral system

Analog data.Analog data.- Describes a device or system that represents Describes a device or system that represents

changing values as continuously variable physical changing values as continuously variable physical quantities. quantities.

- Analog transmissions occur by the changing Analog transmissions occur by the changing amplitude, frequency or phase of the signal to amplitude, frequency or phase of the signal to represent different values.represent different values.

- For example, telephones take sound (audio signal) For example, telephones take sound (audio signal) and turn them into electrical signal that can be and turn them into electrical signal that can be transmitted over traditional telephone lines. transmitted over traditional telephone lines.

Digital Data.Digital Data. - Describes any system based on Describes any system based on

discontinuous data or events. discontinuous data or events. - Describe as a flow of discrete pulses similar Describe as a flow of discrete pulses similar

to the motion of turning a switch off and on. to the motion of turning a switch off and on. - Computers are digital machines because at Computers are digital machines because at

their most basic level they can distinguish their most basic level they can distinguish between just two values, 0 and 1, or off and between just two values, 0 and 1, or off and on. on.

- Example : When you play a compact disc, Example : When you play a compact disc, the CD player reads the digital data, the CD player reads the digital data, translates it back into its original analog translates it back into its original analog form, and sends it to the amplifier and form, and sends it to the amplifier and eventually the speakers. eventually the speakers.

Waveform of analog & digital plotted Waveform of analog & digital plotted overtime.overtime.

Baud rates for digital signals.Baud rates for digital signals.

Baud RateBaud Rate - Baud rate refers to the number of signal units per Baud rate refers to the number of signal units per

second that are required to represent those bits.second that are required to represent those bits.- Determines the bandwidth required to send the signal.Determines the bandwidth required to send the signal.- Baud rate <= (bit rate / number of bits represented by Baud rate <= (bit rate / number of bits represented by

each signal shift).each signal shift). Bit RateBit Rate - Bit rate is the number of bits transmitted during one Bit rate is the number of bits transmitted during one

second.second.- It is quantified using the bit per second (It is quantified using the bit per second (bit/sbit/s) unit. ) unit. - Bit rate = (baud rate* number of bits represented by Bit rate = (baud rate* number of bits represented by

each signal).each signal).

Relationship between Data Rate Relationship between Data Rate and Bandwidthand Bandwidth

Data RateData Ratedata signaling ratedata signaling rate (DSR) is the aggregate rate at (DSR) is the aggregate rate at which data pass a point in the transmission path of a which data pass a point in the transmission path of a data transmission system. data transmission system.

BandwidthBandwidth Analog data signals related to bandwidth.Analog data signals related to bandwidth.- For analog signals, For analog signals, bandwidthbandwidth is the width, usually is the width, usually

measured in hertz, of a frequency band f2 − f1. measured in hertz, of a frequency band f2 − f1. - Define the limitation of analog transmission.Define the limitation of analog transmission.- Relate directly to the volume of data the line can carry Relate directly to the volume of data the line can carry

per time unit.per time unit.- The greater bandwidth, the greater transmission The greater bandwidth, the greater transmission

capacity.capacity.- Bandwidth measured – range of frequency that can Bandwidth measured – range of frequency that can

transmitted on a circuit. transmitted on a circuit.

Digital data signals related to Digital data signals related to bandwidth.bandwidth.

For digital signals and by extension For digital signals and by extension from the above, the word from the above, the word bandwidthbandwidth is also used to mean the amount of is also used to mean the amount of data that can be transferred through a data that can be transferred through a digital connection in a given time digital connection in a given time period (i.e., the connection's period (i.e., the connection's bit ratebit rate). ).

In such cases, bandwidth is usually In such cases, bandwidth is usually measured in bits or bytes per second.measured in bits or bytes per second.

Data signalsData signals ModulationModulation is the process of varying a is the process of varying a carrier signalcarrier signal, ,

typically a sinusoidal signal, in order to use that signal to typically a sinusoidal signal, in order to use that signal to convey information. convey information.

One of the three key characteristics of a signal are One of the three key characteristics of a signal are usually modulated: its phase, frequency or amplitude. usually modulated: its phase, frequency or amplitude.

A device that performs modulation is known as a A device that performs modulation is known as a modulatormodulator and a device that performs demodulation is and a device that performs demodulation is known as a known as a demodulatordemodulator. .

modulator – translate from digital signal to analog signal.modulator – translate from digital signal to analog signal. demodulator – translate from analog signal to digital demodulator – translate from analog signal to digital

signal. signal. The device that is related and being used  to change the The device that is related and being used  to change the

data form is called data form is called MODEM.MODEM. When Modem is use to send binary data over ATL-When Modem is use to send binary data over ATL-

Analog Line Transmission (ATL).Analog Line Transmission (ATL).

ENCODINGENCODING

We must encode data into signals to send We must encode data into signals to send them from one place to another.them from one place to another.

The signal must be manipulated and it The signal must be manipulated and it contains identifiable changes that are contains identifiable changes that are recognizable to the sender and receiver as recognizable to the sender and receiver as representing the information intended.representing the information intended.

Types of encoding:Types of encoding:- Digital to digitalDigital to digital- Analog to digitalAnalog to digital- Digital to analogDigital to analog- Analog to analogAnalog to analog

Digital to digital encodingDigital to digital encoding- The representation of digital information by a The representation of digital information by a

digital signal.digital signal.- E.g. when you transmit data from your E.g. when you transmit data from your

computer to your printer, both the original data computer to your printer, both the original data and the transmitted data are digital.and the transmitted data are digital.

Analog to digital encodingAnalog to digital encoding- The representation of analog information by a The representation of analog information by a

digital signal.digital signal.- E.g. to record a singer’s voice onto a compact E.g. to record a singer’s voice onto a compact

disc, you use digital means to replicate analog disc, you use digital means to replicate analog information.information.

Digital to analog encodingDigital to analog encoding- The representation of digital information by The representation of digital information by

an analog signalan analog signal- E.g. when you transmit data from one E.g. when you transmit data from one

computer to another across a public access computer to another across a public access phone line- modemphone line- modem

Analog to analog encodingAnalog to analog encoding- The representation of analog information by The representation of analog information by

an analog signal.an analog signal.- E.g. the familiar utility; radio station.E.g. the familiar utility; radio station.- Ways of analog to analog encapsulationWays of analog to analog encapsulation . Amplitude modulation – AM. Amplitude modulation – AM

. Frequency modulation – FM. Frequency modulation – FM

. Phase modulation - PM. Phase modulation - PM

Amplitude modulation - AMAmplitude modulation - AM- The carries signal is modulated so that its amplitude varies The carries signal is modulated so that its amplitude varies

with the changing amplitude of the modulating signal.with the changing amplitude of the modulating signal.- The amplitude of the carrier wave is afunction of the The amplitude of the carrier wave is afunction of the

amplitude of the modulating wave.amplitude of the modulating wave.

Frequency modulation – FMFrequency modulation – FM- The frequency of the carrier signal is modulated to follow the The frequency of the carrier signal is modulated to follow the

changing voltage level (amplitude) of the modulating signal.changing voltage level (amplitude) of the modulating signal.- The frequency of the carrier wave is a function of the amplitude The frequency of the carrier wave is a function of the amplitude

of the modulating wave.of the modulating wave.

Phase modulation – PMPhase modulation – PM- Used in some systems as an alternative Used in some systems as an alternative

to frequency modulation.to frequency modulation.- The phase of the carrier signal is The phase of the carrier signal is

modulated to follow the changing modulated to follow the changing voltage level (amplitude) of the voltage level (amplitude) of the modulating signal.modulating signal.

Noise & AttenuationNoise & Attenuation NoiseNoise - Is unwanted additions to optical or electromagnetic Is unwanted additions to optical or electromagnetic

signal.signal.- Lighting and other weather extremes can cause noisy Lighting and other weather extremes can cause noisy

circuits and transmissions error.circuits and transmissions error.- Electronics interference from lights and electronic devices Electronics interference from lights and electronic devices

can cause errorscan cause errors- NoiseNoise can be considered data without meaning; that is, can be considered data without meaning; that is,

data that is not being used to transmit a signal, but is data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other simply produced as an unwanted by-product of other activities activities

- In some cases a little noise may be considered In some cases a little noise may be considered advantageous, allowing a dithered representation of advantageous, allowing a dithered representation of signals below the minimum strength, or between two signals below the minimum strength, or between two quantization levels quantization levels

AttenuationAttenuation

- AttenuationAttenuation is the loss of signal strength over is the loss of signal strength over distancedistance

- Attenuation is the decrease in intensity of a signal, Attenuation is the decrease in intensity of a signal, beam, or wave as a result of absorption of energy beam, or wave as a result of absorption of energy and of scattering out of the path to the detector. and of scattering out of the path to the detector.

- Attenuation is a contributing factor Attenuation is a contributing factor to why cable to why cable designs must specify limits in the lengths of designs must specify limits in the lengths of cable runs. cable runs.

- Attenuation can affect a network because it limits Attenuation can affect a network because it limits the length of network cabling over which you can the length of network cabling over which you can send a messagesend a message.

- When signal strength falls below certain limits, the When signal strength falls below certain limits, the electronic equipment that receives the signal can electronic equipment that receives the signal can experience difficulty isolating the original signal experience difficulty isolating the original signal from the noise present in all electronic from the noise present in all electronic transmissions. transmissions.

Delay DistortionDelay Distortion

Distortion may result from many Distortion may result from many mechanisms. mechanisms.

Examples include nonlinearities in Examples include nonlinearities in the transfer function of an active the transfer function of an active device, such as a vacuum tube, device, such as a vacuum tube, transistor, or operational amplifier. transistor, or operational amplifier.

Distortion may also be caused by a Distortion may also be caused by a passive component such as a passive component such as a coaxial cable or optical fiber, or by coaxial cable or optical fiber, or by inhomogeneities, reflections, inhomogeneities, reflections, etc.,etc., in in the propagation path. the propagation path. 

Categories of NoiseCategories of Noise

Thermal noiseThermal noise Intermodulation Intermodulation

Noise/InterferenceNoise/Interference CrosstalkCrosstalk

Thermal noiseThermal noise - Is the noise generated by the equilibrium - Is the noise generated by the equilibrium

fluctuations of the electric current inside an fluctuations of the electric current inside an electrical conductor, which happens without electrical conductor, which happens without any applied voltage, due to the random any applied voltage, due to the random thermal motion of the charge carriers (the thermal motion of the charge carriers (the electrons).electrons).

- Due to the random motion of electrons is - Due to the random motion of electrons is unavoidable but usually relatively small unavoidable but usually relatively small compared to our signal.compared to our signal.

Intermodulation Noise/InterferenceIntermodulation Noise/Interference - Is the result of two radio signals of Is the result of two radio signals of

different frequencies being mixed different frequencies being mixed together, forming additional signals at together, forming additional signals at frequencies that are not at harmonic frequencies that are not at harmonic frequencies (integer multiples) of either.frequencies (integer multiples) of either.

- The cause for intermodulation is the The cause for intermodulation is the existence of non-linear characteristics existence of non-linear characteristics of the according equipment. of the according equipment.

CrosstalkCrosstalk (NEXT – near-end (NEXT – near-end crosstalk)crosstalk)

- Occur when electrical noise on the Occur when electrical noise on the cable originates from signals on other cable originates from signals on other wires in the cable.wires in the cable.

- When two wires are near each other When two wires are near each other and untwisted, energy from one wire and untwisted, energy from one wire can wind up in an adjacent wire and can wind up in an adjacent wire and vise versa.vise versa.

Note:Note: In telephony, crosstalk is usually In telephony, crosstalk is usually happen when you hearing another happen when you hearing another conversation during your call.conversation during your call.

Transmission Media.Transmission Media.

Coaxial cable Coaxial cable Twisted-pair cable Twisted-pair cable Fiber-optic cableFiber-optic cable

Twisted-pair cableTwisted-pair cable The dominant cable type for all new The dominant cable type for all new

network designs that employ copper cable. network designs that employ copper cable. low cost. low cost. inexpensive to install and offers the lowest inexpensive to install and offers the lowest

cost per foot of any cable type.cost per foot of any cable type. A basic twisted-pair cable consists of two A basic twisted-pair cable consists of two

strands of copper wire twisted together strands of copper wire twisted together

This twisting reduces the tendency of the This twisting reduces the tendency of the cable to radiate radio frequency noise that cable to radiate radio frequency noise that interferes with nearby cables and electronic interferes with nearby cables and electronic components. components.

Twisting also controls the tendency of the Twisting also controls the tendency of the wires in the pair to cause EMI in each other. wires in the pair to cause EMI in each other.

Whenever two wires are in close proximity, Whenever two wires are in close proximity, the signals in each wire tend to produce the signals in each wire tend to produce noise, called crosstalk noise, called crosstalk

Twisting the wires in the pair reduces Twisting the wires in the pair reduces crosstalk in much the same way that twisting crosstalk in much the same way that twisting reduces the tendency of the wires to radiate reduces the tendency of the wires to radiate EMI.EMI.

Two types of twisted-pair cable are used in Two types of twisted-pair cable are used in LANs: shielded and unshielded.LANs: shielded and unshielded.

Shielded twisted-pair(STP)Shielded twisted-pair(STP)

Shielded twisted-pair cabling consists Shielded twisted-pair cabling consists of one or more twisted pairs of of one or more twisted pairs of

cables enclosed in a foil wrap and cables enclosed in a foil wrap and woven copper shielding woven copper shielding

Coaxial and STP cables use shields for the same Coaxial and STP cables use shields for the same purpose. purpose.

The shield is connected to the ground portion of the The shield is connected to the ground portion of the electronic device to which the cable is connected. electronic device to which the cable is connected.

A ground is a portion of the device that serves as an A ground is a portion of the device that serves as an electrical reference point, and usually, it literally is electrical reference point, and usually, it literally is connected to a metal stake driven into the ground. connected to a metal stake driven into the ground.

A properly grounded shield prevents signals from A properly grounded shield prevents signals from getting into or out of the cable.getting into or out of the cable.

Unshielded Twisted-Pair (UTP) CableUnshielded Twisted-Pair (UTP) Cable

Unshielded twisted-pair cable Unshielded twisted-pair cable doesn’t incorporate a braided doesn’t incorporate a braided

shield into its structure. shield into its structure. However, the characteristics of However, the characteristics of

UTP are similar in many ways to UTP are similar in many ways to STP. STP.

several twisted-pairs can be bundled several twisted-pairs can be bundled together in a single cable. together in a single cable.

These pairs typically are color coded to These pairs typically are color coded to distinguish them.distinguish them.

Telephone systems commonly use UTP Telephone systems commonly use UTP cabling. cabling.

UTP cable is a latecomer to high-UTP cable is a latecomer to high-performance LANs because engineers only performance LANs because engineers only recently solved the problems of managing recently solved the problems of managing radiated noise and susceptibility to EMI. radiated noise and susceptibility to EMI.

UTP cable is available in the following five grades, or UTP cable is available in the following five grades, or categories:categories:

Categories 1 and 2. Categories 1 and 2. - Voice-grade cables (telephone systems) - Voice-grade cables (telephone systems) - suitable only for voice and for low data rates (below 4 Mbps). - suitable only for voice and for low data rates (below 4 Mbps).

Category 3. Category 3. - The lowest data-grade cable, this type of cable The lowest data-grade cable, this type of cable - suited for data rates up to 10 Mbps. suited for data rates up to 10 Mbps. - Enable the cable to support data rates up to 100 Mbps. Enable the cable to support data rates up to 100 Mbps. - Category 3, which uses for most telephone installations. Category 3, which uses for most telephone installations.

Category 4. Category 4. - This data-grade cable, which consists of four twisted-pairs, is - This data-grade cable, which consists of four twisted-pairs, is

suitable for data rates up to 16 Mbps. suitable for data rates up to 16 Mbps.

Category 5. Category 5. - This data-grade cable, which also consists of four twisted-pairs, is This data-grade cable, which also consists of four twisted-pairs, is

suitable for data rates up to 100 Mbps. suitable for data rates up to 100 Mbps. - Most new cabling systems for 100 Mbps data rates are designed Most new cabling systems for 100 Mbps data rates are designed

around Category 5 cable.around Category 5 cable.

Coaxial cablesCoaxial cables

Coaxial cables were the first cable types used Coaxial cables were the first cable types used in LANs.in LANs.

Coaxial cable gets its name because two Coaxial cable gets its name because two conductors share a common axis; the conductors share a common axis; the

cable is most frequently referred to as a cable is most frequently referred to as a coax.coax.

The components of a coaxial cable are as follows:The components of a coaxial cable are as follows:

A A centercenter conductorconductor, , - although usually solid copper wire, sometimes is - although usually solid copper wire, sometimes is

made of stranded wire. made of stranded wire. An An outerouter conductorconductor - forms a tube surrounding the center conductor.forms a tube surrounding the center conductor.- This conductor can consist of braided wires, This conductor can consist of braided wires,

metallic foil, or both. metallic foil, or both. - The outer conductor, frequently called the shield, The outer conductor, frequently called the shield,

serves as a ground and also protects the inner serves as a ground and also protects the inner conductor from EMI. conductor from EMI.

An An insulationinsulation layerlayer - keeps the outer conductor spaced evenly from the - keeps the outer conductor spaced evenly from the

inner conductor. inner conductor. A plastic encasement (jacket) protects the cable A plastic encasement (jacket) protects the cable

from damage.from damage.  

Fiber-Optic Cable.Fiber-Optic Cable.

Fiber-optic cable is the ideal cable Fiber-optic cable is the ideal cable for data transmission. for data transmission.

This type of cable accommodate This type of cable accommodate extremely high bandwidths.extremely high bandwidths.

The center conductor of a fiber-optic cable is a fiber The center conductor of a fiber-optic cable is a fiber that consists of highly refined glass or plastic that consists of highly refined glass or plastic designed to transmit light signals with little loss. designed to transmit light signals with little loss.

A glass core supports a longer cabling distance, but A glass core supports a longer cabling distance, but a plastic core is typically easier to work with. a plastic core is typically easier to work with.

The fiber is coated with a cladding that reflects The fiber is coated with a cladding that reflects signals back into the fiber to reduce signal loss.signals back into the fiber to reduce signal loss.

A plastic sheath protects the fiber. A plastic sheath protects the fiber. The two disadvantages of fiber-optic, however, are The two disadvantages of fiber-optic, however, are

cost and installation difficulty.cost and installation difficulty.

UTPUTP- - 100 Mbps.100 Mbps.

STPSTP- - 155 Mbps with 100-meter cable runs. 155 Mbps with 100-meter cable runs.

- 16 Mbps, top data rate for Token Ring networks.- 16 Mbps, top data rate for Token Ring networks. CoaxialCoaxial

- - bandwidth between 2.5 Mbps (ARCnet) and 10 Mbps bandwidth between 2.5 Mbps (ARCnet) and 10 Mbps (Ethernet). (Ethernet). -Thicker coaxial cables offer higher bandwidth-Thicker coaxial cables offer higher bandwidth - potential bandwidth of coaxial is much higher than 10 - potential bandwidth of coaxial is much higher than 10

Mbps. Mbps. - no more use in LAN today- no more use in LAN today

   Fiber-OpticFiber-Optic

- - support high data rates (as high as 200,000 Mbps) with support high data rates (as high as 200,000 Mbps) with long cable runs.long cable runs.

- can transmit 100 Mbps signals for several kilometers. - can transmit 100 Mbps signals for several kilometers.

Wireless alternativesWireless alternatives MicrowavesMicrowaves- MicrowavesMicrowaves are electromagnetic waves with are electromagnetic waves with

wavelengths longer than those of infrared light, wavelengths longer than those of infrared light, but shorter than those radio waves. but shorter than those radio waves.

- Microwaves, also known as Microwaves, also known as super-high super-high frequency (SHF)frequency (SHF) signals, have wavelengths signals, have wavelengths approximately in the range of 30 cm (frequency approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). = 1 GHz) to 1 mm (300 GHz).

- Types of microwavesTypes of microwaves * Terrestrial microwave * Terrestrial microwave * Satellite microwave * Satellite microwave

Terrestrial microwavesTerrestrial microwaves

- Terrestrial microwave communication employs Earth-based Terrestrial microwave communication employs Earth-based transmitters and receivers. transmitters and receivers.

- The frequencies used are in the low-gigahertz range, which The frequencies used are in the low-gigahertz range, which limits all communications to line-of-sight and require line-of-limits all communications to line-of-sight and require line-of-sight transmission and reception equipment. sight transmission and reception equipment.

- You probably have seen terrestrial microwave equipment in You probably have seen terrestrial microwave equipment in the form of telephone relay towers, which are placed every the form of telephone relay towers, which are placed every few miles to relay telephone signals crosscountry.few miles to relay telephone signals crosscountry.

- Antenna are mounted on towers that are in turn often Antenna are mounted on towers that are in turn often mounted on hills or mountains.mounted on hills or mountains.

- Microwave transmissions typically use a parabolic antenna Microwave transmissions typically use a parabolic antenna that produces a narrow, highly directional signal. that produces a narrow, highly directional signal.

- A similar antenna at the receiving site is sensitive to signals A similar antenna at the receiving site is sensitive to signals only within a narrow focus only within a narrow focus

- Microwave systems are highly susceptible to atmospheric Microwave systems are highly susceptible to atmospheric interference and also can be vulnerable to electronic interference and also can be vulnerable to electronic eavesdropping. eavesdropping.

- For this reason, signals transmitted through microwave are For this reason, signals transmitted through microwave are frequently encrypted. frequently encrypted.

Satellite microwavesSatellite microwaves

- Satellite transmission is much like line-of-sight microwave transmission Satellite transmission is much like line-of-sight microwave transmission in which one of the stations is a satellite orbiting the earth.in which one of the stations is a satellite orbiting the earth.

- Satellites orbiting at 22,300 miles distance remain located above a fixed Satellites orbiting at 22,300 miles distance remain located above a fixed point on earth.point on earth.

- Satellite microwave can provide transmission capability to and from Satellite microwave can provide transmission capability to and from location on earth, no matter how remote.location on earth, no matter how remote.

- Earth stations use parabolic antennas (satellite dishes) to communicate Earth stations use parabolic antennas (satellite dishes) to communicate with satellites. with satellites.

- These satellites then can retransmit signals in broad or narrow beams, These satellites then can retransmit signals in broad or narrow beams, depending on the locations set to receive the signals.depending on the locations set to receive the signals.

- When the destination is on the opposite side of the earth, for example, When the destination is on the opposite side of the earth, for example, the first satellite cannot transmit directly to the receiver and thus must the first satellite cannot transmit directly to the receiver and thus must relay the signal through another satellite.relay the signal through another satellite.

- Because no cables are required, satellite microwave communication is Because no cables are required, satellite microwave communication is possible with most remote sites and with mobile devices, which enables possible with most remote sites and with mobile devices, which enables transmission with ships at sea and motor vehicles.transmission with ships at sea and motor vehicles.

- Unfortunately, satellite communication is extremely expensive. Building Unfortunately, satellite communication is extremely expensive. Building and launching a satellite can cost easily in excess of a billion dollars. and launching a satellite can cost easily in excess of a billion dollars.

Application Of Infrared TechnologyApplication Of Infrared Technology..

Night VisionNight Vision Other Imaging Other Imaging ThermographyThermography

Heating Heating Communications Communications

Spectroscopy Spectroscopy

Night VisionNight Vision

Infrared is used in night-vision equipment, Infrared is used in night-vision equipment, when there is insufficient visible light to see when there is insufficient visible light to see an object. an object.

The radiation is detected and turned into an The radiation is detected and turned into an image on a screen, hotter objects showing image on a screen, hotter objects showing up brighter, enabling the police and military up brighter, enabling the police and military to acquire thermally significant targets, such to acquire thermally significant targets, such as human beings and automobiles.as human beings and automobiles.

Other Imaging Other Imaging

In infrared photography, infrared filters are used to In infrared photography, infrared filters are used to capture only the infrared spectrum. capture only the infrared spectrum.

Digital cameras often use infrared blockers.Digital cameras often use infrared blockers. Cheaper digital cameras and some camera Cheaper digital cameras and some camera

phones which do not have appropriate filters can phones which do not have appropriate filters can "see" infrared, appearing as a bright white colour "see" infrared, appearing as a bright white colour (try pointing a TV remote at your digital camera).(try pointing a TV remote at your digital camera).

This is especially pronounced when taking This is especially pronounced when taking pictures of subjects near bright areas (such as pictures of subjects near bright areas (such as near a lamp), where the resulting infrared near a lamp), where the resulting infrared interference can wash out the image.interference can wash out the image.

ThermographyThermography

Infrared radiation can be used to remotely Infrared radiation can be used to remotely determine the temperature of objects determine the temperature of objects

This is termed thermography, or in the case of This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed very hot objects in the NIR or visible it is termed pyrometry. pyrometry.

Thermography (thermal imaging) is mainly used in Thermography (thermal imaging) is mainly used in military and industrial applications but the military and industrial applications but the technology is reaching the public market in the technology is reaching the public market in the form of infrared cameras on cars due to the form of infrared cameras on cars due to the massively reduced production costsmassively reduced production costs

HeatingHeating

Infrared radiation is used in Infrared saunas Infrared radiation is used in Infrared saunas to heat the sauna's occupants and to to heat the sauna's occupants and to remove ice from the wings of aircraft (de-remove ice from the wings of aircraft (de-icing).icing).

CommunicationsCommunications

IR data transmission is also employed in short-range IR data transmission is also employed in short-range communication among computer peripherals and personal communication among computer peripherals and personal digital assistants. digital assistants.

These devices usually conform to standards published by These devices usually conform to standards published by IrDA, the Infrared Data Association. Remote controls and IrDA, the Infrared Data Association. Remote controls and IrDA devices use infrared light-emitting diodes (LEDs) to IrDA devices use infrared light-emitting diodes (LEDs) to emit infrared radiation which is focused by a plastic lens emit infrared radiation which is focused by a plastic lens into a narrow beam. into a narrow beam.

The beam is modulated, i.e. switched on and off, to encode The beam is modulated, i.e. switched on and off, to encode the data. the data.

The receiver uses a silicon photodiode to convert the The receiver uses a silicon photodiode to convert the infrared radiation to an electric current. infrared radiation to an electric current.

It responds only to the rapidly pulsing signal created by the It responds only to the rapidly pulsing signal created by the transmitter, and filters out slowly changing infrared transmitter, and filters out slowly changing infrared radiation from ambient light. radiation from ambient light.

Spectroscopy Spectroscopy

Infrared radiation spectroscopy is the study of the Infrared radiation spectroscopy is the study of the composition of (usually) organic compounds, composition of (usually) organic compounds, finding out a compound's structure and finding out a compound's structure and composition based on the percent transmittance composition based on the percent transmittance of IR radiation through a sample. of IR radiation through a sample.

Different frequencies are absorbed by different Different frequencies are absorbed by different stretches and bends in the molecular bonds stretches and bends in the molecular bonds occurring inside the sample. occurring inside the sample.

Carbon dioxide, for example, has an absorption Carbon dioxide, for example, has an absorption band at 4.2µm.band at 4.2µm.

Data Communication Data Communication InterfaceInterface

DTE interfaceDTE interface DCE interfaceDCE interface

Data Terminal Equipment (DTE)Data Terminal Equipment (DTE) Includes any unit that functions either as a source of or as Includes any unit that functions either as a source of or as

a destination for binary digital data. a destination for binary digital data. The data terminal equipment (DTE) may be a single piece The data terminal equipment (DTE) may be a single piece

of equipment or an interconnected subsystem of multiple of equipment or an interconnected subsystem of multiple pieces of equipment that perform all the required functions pieces of equipment that perform all the required functions necessary to permit users to communicate.necessary to permit users to communicate.

A user interacts with the DTE, or the DTE may be the user. A user interacts with the DTE, or the DTE may be the user. The DTE interacts with the data circuit-terminating The DTE interacts with the data circuit-terminating equipment (DCE).equipment (DCE).

Data Circuit-Terminating EquipmentData Circuit-Terminating Equipment ((DCE)DCE)

Includes any functional unit that transmits or Includes any functional unit that transmits or receives data in the form of an analog or digital receives data in the form of an analog or digital signal through a network. signal through a network.

Data Communications Equipment (DCE) is a Data Communications Equipment (DCE) is a device that communicates with a Data Terminal device that communicates with a Data Terminal Equipment (DTE) device in RS-232C Equipment (DTE) device in RS-232C communications.communications.

Usually, the DTE device is the terminal (or Usually, the DTE device is the terminal (or computer), and the DCE is a modem.computer), and the DCE is a modem.

When two devices that are both DTE or both DCE When two devices that are both DTE or both DCE that must be connected together without a modem that must be connected together without a modem or a similar media translater between them, a or a similar media translater between them, a NULL modem must be used NULL modem must be used   

MultiplexingMultiplexing

Whenever the transmission capacity of a Whenever the transmission capacity of a medium linking two devices is greater than medium linking two devices is greater than the transmission needs of the devices, the the transmission needs of the devices, the link can be shared, much as a large water link can be shared, much as a large water pipe can carry water to several separate pipe can carry water to several separate houses at once. houses at once.

DefinitionDefinition

is a device for taking several is a device for taking several separate digital data streams separate digital data streams and combining them together and combining them together into one data stream of a higher into one data stream of a higher data rate. data rate.

It also the set of techniques that It also the set of techniques that allows the simultaneous allows the simultaneous transmission of multiple signals transmission of multiple signals across a single data link.across a single data link.

Types of multiplexed Types of multiplexed techniquestechniques

FDM – frequency division FDM – frequency division multiplexingmultiplexing

TDM – time division TDM – time division multiplexingmultiplexing

FDMFDM Is an analog technique that can be applied when Is an analog technique that can be applied when

the bandwidth of a link is greater than the the bandwidth of a link is greater than the combined bandwidths of the signals to be combined bandwidths of the signals to be transmitted. transmitted.

In FDM, signals generated by each sending In FDM, signals generated by each sending device modulate different carrier frequencies.device modulate different carrier frequencies.

Each signal modulates a different carrier Each signal modulates a different carrier frequency. The modulated carriers are combined frequency. The modulated carriers are combined to form a new signal that is then sent across the to form a new signal that is then sent across the linklink

The multiplexers modulate and combine signals The multiplexers modulate and combine signals while demultiplexers decompose and demodulate.while demultiplexers decompose and demodulate.

Application of FDMApplication of FDM

FDM can be used in broadband LANs (a FDM can be used in broadband LANs (a standard for Ethernet standard for Ethernet

Familiar application of FDM is a cable Familiar application of FDM is a cable television. The coaxial cable used in a television. The coaxial cable used in a cable television system has a bandwidth of cable television system has a bandwidth of approximately 500 MHz. An individual approximately 500 MHz. An individual television channel requires about 6 MHz of television channel requires about 6 MHz of bandwidth for transmission. The coaxial bandwidth for transmission. The coaxial cable, therefore can carry many cable, therefore can carry many multiplexed channels. A demultiplexer at multiplexed channels. A demultiplexer at your television allows you to select which of your television allows you to select which of those channels you wish to receive.those channels you wish to receive.

TDMTDM

Is a digital process that can be applied Is a digital process that can be applied when the data rate capacity of the when the data rate capacity of the transmission medium is greater than the transmission medium is greater than the data rate required data rate required

TDM can be classified as :TDM can be classified as :

- synchronous- synchronous

- asynchronous- asynchronous

Synchronous TDMSynchronous TDM

- - each frame contains at least one time slot each frame contains at least one time slot dedicated to each device. The order in dedicated to each device. The order in

which which each device sends its data to the each device sends its data to the frame is frame is unvarying. If the device has no data unvarying. If the device has no data to send, to send, its time slot is sent empty.its time slot is sent empty.

- - a bit may be added to the beginning of a bit may be added to the beginning of each each frame for synchronization.frame for synchronization.

Asynchronous TDMAsynchronous TDM

-- the time slot order of a frame depends the time slot order of a frame depends on which devices have data to send at on which devices have data to send at that time.that time.

-- asynchronous TDM adds device asynchronous TDM adds device addresses to each time slot.addresses to each time slot.